Abstract: The matter quantities and the value of air purification by vegetation in Xi'an City are estimated．The results show that the total value of air parification by vegetation is 3426．356 million yuan per year．which is 6％ of GDP of Xi’an in 1999；the value of fixing CO₂ and releasing O₂ is 2718．309 million yuan per year，the value of absorbing the three pollutants，SO₂、HF、NO_x，is 182．896 million yuan per year，the value of dust retention is 182．511 million yuan per year，the value of sterilization is 342．640 million yuan per year．Using the function of overlaying analysis on Maplnfo 6．0，the statistical results of each cantonal are gained．